Apparatus and method for combining analog and digital automatic gain control in receivers with digital signal processing

Abstract

Two automatic gain control (AGC) loops are connected by a feedforward signal. An intermediate frequency (IF) filter may be located between the output of the front-end loop and the input of the back-end loop. Stability and responsiveness are improved because neither AGC loop includes a narrowband filter. The front-end loop may include an analog gain-controlled element, but the remainder of the invention may be implemented digitally. The front-end loop prevents overloading of the A/D converter that feeds the gain-controlled signal to the remainder of the invention, and the back-end loop compensates for actions taken in the front-end loop.

I claim: 
     
       1. An automatic gain control system, comprising: a first automatic gain control (AGC) loop for receiving an input signal, for producing a first gain controlled-signal, and for producing a first control signal in response to said first gain-controlled signal, said first gain-controlled signal produced in response to said input signal and said first control signal;   a filter for filtering said first gain-controlled signal and for providing a filtered first gain-controlled signal; and   a second automatic gain control (AGC) loop for producing a second gain-controlled signal and for producing a second control signal in response to said second gain-controlled signal and said first control signal, said second gain-controlled signal produced in response to said filtered first gain-controlled signal and said second control signal.   
     
     
       2. The automatic gain control system recited in claim 1, wherein said first AGC loop comprises: an analog gain-controlled amplifier for producing said first gain-controlled signal in response to said input signal and said first control signal; and   an analog-to-digital (A/D) converter connected to said gain-controlled amplifier for receiving said first gain-controlled signal and for producing a digital first gain-controlled signal.   
     
     
       3. The automatic gain control system recited in claim 2, wherein said first AGC loop further comprises a digital-to-analog (D/A) converter connected to said first gain-controlled amplifier for controlling the gain of said gain-controlled amplifier in response to said first control signal. 
     
     
       4. The automatic gain control system recited in claim 2, wherein said first AGC loop further comprises a first amplitude modulation (AM) detector for receiving said digital first gain-controlled signal and for producing a first signal level estimate. 
     
     
       5. The automatic gain control system recited in claim 4, wherein said first AGC loop further comprises a first amplitude error corrector for producing a first correction signal in response to said first signal level estimate and a first predetermined reference signal. 
     
     
       6. The automatic gain control system recited in claim 5, wherein said first amplitude error corrector comprises a first summer for producing a first difference signal equal to the difference between said first signal level estimate and said first predetermined reference signal. 
     
     
       7. The automatic gain control system recited in claim 6, wherein: said first AGC loop further comprises tailoring for producing a first tailored correction signal;   said first tailored correction signal is produced in response to a first function of said first difference signal when said first AGC loop is in a decay mode; and   said first tailored correction signal is produced in response to a second function of said first difference signal when said first AGC loop is in an attack mode.   
     
     
       8. The automatic gain control system recited in claim 7, wherein: said first function has a constant decay gain greater than one; and   said second function has a constant attack gain greater than one only between a value of said first tailored correction signal equal to zero and a value of said first tailored correction signal equal to a predetermined correction limit.   
     
     
       9. The automatic gain control system recited in claim 7, wherein said first AGC loop further comprises: an integrator for integrating said first tailored correction signal; and   a limiter for limiting said integrated first tailored correction signal.   
     
     
       10. The automatic gain control system recited in claim 7, wherein said first AGC loop further comprises a linearizer for linearizing said first gain-controlled signal with respect to an exponential function of said first control signal. 
     
     
       11. The automatic gain control system recited in claim 1, wherein said second AGC loop comprises a digital multiplier for producing said first gain-controlled signal in response to said first gain-controlled signal and said second control signal. 
     
     
       12. The automatic gain control system recited in claim 11, wherein said second AGC loop further comprises: total gain means for producing a total gain signal in response to said second gain-controlled signal; and   summing means for producing said second control signal in response to the difference between said total gain signal and said first control signal.   
     
     
       13. The automatic gain control system recited in claim 12, wherein said total gain means comprises a second amplitude modulation (AM) detector for receiving said digital second gain-controlled signal and for producing a second signal level estimate. 
     
     
       14. The automatic gain control system recited in claim 13, wherein said total gain means further comprises a second amplitude error corrector for producing a second correction signal in response to said second signal level estimate and a second predetermined reference signal. 
     
     
       15. The automatic gain control system recited in claim 14, wherein said second amplitude error corrector comprises summing means for producing a second difference signal equal to the difference between said second signal level estimate and said second predetermined reference signal. 
     
     
       16. The automatic gain control system recited in claim 14, wherein: said total gain means further comprises tailoring means for producing a second tailored correction signal;   said second tailored correction signal is produced in response to a first function of said second difference signal when said second AGC loop is in a decay mode; and   said second tailored correction signal is produced in response to a second function of said second difference signal when said second AGC loop is in an attack mode.   
     
     
       17. The automatic gain control system recited in claim 16, wherein: said first function has a constant decay gain greater than one; and   said second function has a constant attack gain greater than one only between a value of said second tailored error correction signal equal to zero and a value of said second tailored error correction signal equal to a predetermined correction limit.   
     
     
       18. The automatic gain control system recited in claim 16, wherein said total gain means further comprises: an integrator for integrating said second tailored correction signal; and   a limiter for limiting said integrated second tailored correction signal.   
     
     
       19. The automatic gain control system recited in claim 16, wherein said total gain means further comprises scaling means for linearizing said second gain-controlled signal with respect to an exponential function of said second control signal. 
     
     
       20. The automatic gain control system recited in claim 19, wherein said exponential function is 2 x , and wherein x is said second control signal. 
     
     
       21. An automatic gain control system, comprising: a first automatic gain control (AGC) loop having a bandwidth, said first AGC loop for receiving a radio frequency input signal and for producing an intermediate frequency (IF) first gain-controlled signal;   an IF filter having a bandwidth wider than said bandwidth of said first AGC loop, said IF filter for receiving said first gain-controlled signal and for producing a filtered first gain-controlled signal; and   a second automatic gain control (AGC) loop having a bandwidth narrower than said bandwidth of said first AGC loop, said second AGC loop for receiving said filtered first gain-controlled signal and for producing a second gain-controlled signal.   
     
     
       22. The automatic gain control system recited in claim 21, further comprising a demodulator for producing a demodulated output signal in response to said second gain-controlled signal. 
     
     
       23. The automatic gain control system recited in claim 21, wherein: said narrowband IF filter is a digital filter;   said first AGC loop comprises analog gain-controlled means;   said first AGC loop produces a first control signal in response to said first gain-controlled signal;   said first AGC loop produces said first gain-controlled signal produced in response to said input signal and said first control signal;   said second AGC loop comprises digital gain-controlled means;   said second AGC loop produces a second control signal in response to said second gain-controlled signal and said first control signal; and   said second AGC loop produces said gain-controlled signal produced in response to said first gain-controlled signal and said second control signal.   
     
     
       24. The automatic gain control system recited in claim 23, wherein said first and second AGC loops each comprise: amplitude modulation (AM) detection means connected to said gain-controlled means of said respective AGC loop;   amplitude error correction means connected to said AM detection means;   tailoring means connected to said amplitude error correction means;   limited integrator means connected to said tailoring means; and   linearizer means connected to said limited integrator means.   
     
     
       25. A method for providing automatic gain control in a receiver having digital signal processing circuitry, comprising the steps of: detecting the amplitude of an input signal and producing a first gain signal in response to said detection;   controlling said amplitude of said input signal in response to said first gain signal;   providing a digital signal having said controlled amplitude to a digital intermediate frequency (IF) filter;   detecting the amplitude of a filtered signal and producing a second gain signal in response to said detection; and   controlling said amplitude of said filtered signal in response to said first and second gain signals.   
     
     
       26. The method for providing automatic gain control recited in claim 25, wherein: said step of controlling said amplitude of said input signal comprises the step of providing said first gain signal to an analog gain-controlled amplifier; and   said step of controlling said amplitude of said filtered signal comprises the step of providing said second gain signal to a digital multiplier.   
     
     
       27. The method for providing automatic gain control recited in claim 26, wherein each of said detecting steps comprises the steps of: detecting an amplitude; and   forming the difference between said amplitude and a reference signal.   
     
     
       28. The method for providing automatic gain control recited in claim 27, wherein each of said detecting steps further comprises the step of tailoring said difference. 
     
     
       29. The method for providing automatic gain control recited in claim 28, wherein each of said detecting steps comprises the step of integrating said tailored difference. 
     
     
       30. The method for providing automatic gain control recited in claim 29, wherein each of said detecting steps further comprises the step of linearizing the respective one of said gain signals in response to said tailored and integrated difference.